1. Field of the Invention
The present invention relates to a vacuum pump that includes a screw groove exhaust unit having a dual exhaust path on inner and outer peripheries.
2. Description of the Related Art
Conventionally, there have been used, for example, vacuum pumps such as a turbo-molecular pump as means for discharging gas inside a process chamber in the process of performing processing inside a high-vacuum process chamber such as a dry etching process or a CVD process in a semiconductor manufacturing process.
In this type of process, a large amount of gas is supplied into a process chamber to increase the speed of the process. In order to improve exhaust performance for discharging a large amount of gas, there is known a structure in which screw stators are arranged on both the outer side and the inner side of a rotor.
An example of conventional turbo-molecular pumps has the following structure.
A rotor includes a rotor shaft and a rotor cylindrical section. A central cylindrical section is disposed on a base. An opening through which the rotor shaft is inserted is formed on the central cylindrical section. An outer screw stator fixed to the base is arranged on the outer side of the rotor cylindrical section. An inner screw stator is arranged on the inner side of the rotor cylindrical section. A through hole for gas path communication which allows the outer screw stator and the inner screw stator to communicate with each other is formed on the rotor cylindrical section. The rotor is driven by a motor which includes a motor rotor disposed on the rotor shaft and a motor stator disposed on the central cylindrical section.
Although not illustrated in WO 2012/032863 A, a cooling jacket for cooling the motor stator is typically disposed on the base. In the above structure, the inner stator is cooled together with the motor stator, and reaction products may be disadvantageously accumulated on the inner screw stator accordingly. When reaction products are accumulated, the diameter of the rotor cylindrical section that has increased due to centrifugal force during driving the pump may be reduced at the time of stopping the pump, and the rotor cylindrical section may adhere to the reaction products accumulated on the inner screw stator, thereby causing trouble in restart of the pump.
When a large amount of gas is discharged when intermittently discharging gas or immediately after starting up the vacuum apparatus, the temperature of the motor stator increases. Thus, the motor stator is cooled by driving the cooling jacket disposed on the lower part of the base. At this point, reaction products rapidly increase, and a large amount of reaction products are accumulated in a gap between the rotor cylindrical section and the inner screw stator. As a result, the pump may be disadvantageously stopped.